Telecommunication system subscribers use speech quality as a benchmark for assessing the overall quality of a network. Regardless of whether perceived speech quality is a subjective judgment, it is one key to retaining subscriber loyalty. For this reason, the removal of hybrid and acoustic echo inherent within telecommunication systems is important for improving perceived voice quality.
The first echo suppression devices were introduced in the 1950s. These systems, used to manage echo generated in satellite-based systems, were essentially voice-activated switches that transmitted a voice path and then were turned off to block any echo signal. Although echo suppressors reduced echo caused by transmission problems in the network, they also resulted in choppy first syllables and artificial volume adjustment. In addition, they eliminated double-talk capabilities, greatly reducing the ability to achieve natural conversations.
Echo cancellation theory was developed in the early 1960s by AT&T Bell Labs followed by the introduction of the first echo-cancellation system in the late 1960s by Communications Satellite (COMSAT) TeleSystems (previously a division of COMSAT Laboratories, hereafter COMSAT). COMSAT designed the first analog echo-canceller systems to demonstrate the feasibility and performance of satellite communications networks. These early echo-cancellation systems were implemented across satellite communication networks to demonstrate the network's performance for long-distance telephony. These systems were not commercially viable because of their size and manufacturing costs.
In the late 1970s, COMSAT developed and sold the first commercial analog-echo cancellers, which were mainly digital devices with an analog interface with the network. The semiconductor revolution of the early 1980s marked the switch from analog to digital communications networks. Soon, multi-channel echo-cancellers with digital interfaces were developed to address new echo problems associated with long-distance digital telephony systems. Based on application-specific integrated circuit (ASIC) technology, these new echo cancellers used high-speed digital signal processing techniques to model and subtract the echo from the echo return path.
Explosive growth in digital and wireless networks have brought to market new analog and digital wireless handsets, numerous service carriers, and new digital network communication protocols such as time division multiple access (TDMA), code division multiple access (CDMA), the global system for mobile communications (GSM), voice over Internet protocol (VOIP), etc. Understanding and overcoming inherent echo problems associated with digital networks will enable telecommunication service providers to achieve superior voice transmission quality that service subscribers expect.
Acoustic echo is generated with analog and digital handsets. This form of echo is produced by poor voice coupling between the earpiece and the microphone in handsets and hands-free devices. Further voice degradation is caused as voice-compressing encoding/decoding devices (vocoders) process the voice paths within the handsets and in wireless networks. This results in returned echo signals with highly variable properties. When compounded with inherent digital transmission delays call quality is greatly diminished.
Hybrid echo is the primary source of echo generated in the public-switched telephone network (PSTN). This electrically generated echo is created as voice signals are transmitted across the network via the hybrid connection at the two-wire/four-wire PSTN conversion points, reflecting electrical energy back to the speaker from the four-wire circuit. Although not a factor in pure digital cellular networks, hybrid echo is problematic in calls that traverse the PSTN. The hybrid is by its nature a leaky device. As voice signals pass from the four-wire to the two-wire portion of the network, the energy in the four-wire section is reflected back on itself, creating the echoed speech. Provided that the round-trip delay occurs within a few milliseconds (i.e., within approximately 28 ms), it generates a sense that the call is live by adding side tones, which contributes positively to the quality of the call.
In cases where the total network delay exceeds approximately 36 ms, however, the positive benefits disappear and intrusive echo results. The amount of signal that is reflected back depends on how well the hybrid matches the two-wire line. In the vast majority of cases, the match is poor, resulting in a considerable level of reflected signal.
Digital processing delays and speech-compression techniques further contribute to degraded voice quality in digital networks. Delays are encountered as signals are processed through various routes using different media within the networks, including copper wire, fiber optic lines, microwave links, international gateways, and satellite signal transmissions. This is especially true with mixed technology networks, where calls are processed across numerous network infrastructures.
Therefore, further improvements to systems and methods for echo cancellation are desired.